Induced airflow from the flow of material is a primary cause of airborne dust around conveyor to conveyor transfer chutes. As air follows the bulk material stream through the transfer chute, it accelerates and generates a pressure gradient in the air. The resulting airflow mixes with the material dust particles, causing dust-laden air to escape and be released to the surrounding area around the transfer point, thereby creating a dusty environment emanating at the exit of the chute.
Currently, dust-laden air is attempted to be actively captured with dust collection, suppression, fogging, or stilling/recirculation chambers. To minimize maintenance and operation costs, passive dust control technologies have been developed in recent years. New engineered controlled flow transfer chutes have been developed following the principal of condensing the flow of material and therefore minimizing the air inside the material stream. These types of controlled flow chutes have been successfully installed in industrial plants without the need for additional dust control systems.
The theory of condensing the material stream works well as long as the cross section of the material equals the cross section of the chute and minimizes or eliminates the boundary layer of induced air. If these currently used transfer chutes encounter a large variation in volume of material loading, they have difficulties maintaining a dust-free environment because the chute cannot maintain a material cross-section equal to the fixed chute cross-section. A gap may form between the material and the inner chute thereby providing a pathway for the induced air to rush through the transfer system, creating a cloud of dust emission at the exit point. Alternatively, the chute may become plugged if the material flow is greater than the chute capacity, resulting in an unscheduled shutdown of the conveyor transfer system.
Currently, there exists no other known commercially available technology that can adequately handle varying material flows while minimizing induced airflow and dust in the work area as well as minimizing the risk of plugging. Some known chutes have difficulties in passively controlling dust-laden air if there are varying material flows. If the chutes are designed for a specific load, and the plant runs at that specific load consistently, the chute typically has few problems. However, if there are varying loads, then there can be dusting issues. Basically, some known commercial prior art chutes actively manipulate the material cross sectional area as well as the material flow, which can be problematic and can result in plugging.
Another One known system has recirculation, or stilling, chambers, which has difficulty handling varying material flow and can be physically or commercially unviable due to the size of the needed chambers. A chamber is placed over and around the exit conveyor in an attempt to minimize the dust created by the induced airflow. However, this chamber is not always an effective way to minimize the dust because the air that goes into the chamber mixes with the bulk material and must exit from the chamber one way or another.
Another example of a known prior art chute involves a uniquely-shaped chute that slows the material flow down so the chute cross section is filled with material as the material travels through the chute. This chute works well with a constant flow of material, but it does not always work well if the flowing material is too sticky because sticky material causes the chute to get plugged.
Another example of prior art is flexible skirt curtains used interior to skirt enclosures located on the receiving belt after the chute discharges the material onto the receiving belt. The curtains are used to restrict flow of air though the transfer system. The rubber skirt curtains typically include slits and drilled holes to allow them to bend and distort with the varying material flow. The curtains are flexible and are un-dampered allowing them to distort and bouncing; with the result that large gaps between material, curtain, and side walls are available for dust laden air to escape the transfer system. The location of the skirt curtain is also above the receiving belt, which can also flex and sag depending on operation.
One other known prior art example has a hopper where the material flows into curved chute through a funnel. If the correct amount of material is flowing, it will be, just enough to fill the funnel and then fill the chute, and there will be no dust at the exit of the chute because there is no air that is allowed into the chute. If there is a lower material flow, then the chute can be physically adjusted up so that the material flow slows down until the chute is allowed to fill up, thereby allowing no air and ultimately no dust exiting the chute. However, it is difficult and time-consuming to move the chute, the variation of material flow is not easily handled by this system, and the chute can potentially plug if not adjusted swiftly enough.
Before the instant invention, another type of dust control material transfer system was developed and patented (U.S. Pat. No. 7,789,217). This system was designed to overcome the issue of varying material capacities by introducing an automated or manually adjustable chute wail system to automatically minimize the gap between the material and the chute wall, thereby creating a full cross-section of material inside the chute at all times. The challenge was to continually adjust the cross section and provide a full material cross-section in the chute. Typically, a chute is designed for a specific maximum capacity, i.e., 2,000 tons per hour (TPH). As long as the incoming conveyor provides 2,000 TPH of material, the chute will have a full cross-section and the induced air will be generally blocked from rushing through the transfer system. However, in a typical conveyor system, the material capacities are rarely constant. Therefore, the chute cross-section needs to be adjusted constantly or it will not always be filled, and a gap may be created to provide the induced airflow a pathway to rush through the transfer system. In addition, the system was supposed to minimize the risk of plugged chutes. In wet and cohesive conditions, the chute can be opened up to provide maximum throughput capacity for sticky materials in the wintertime or wet season. Typically, dusting is not an issue during wet seasons.
The underlying principle of the system in U.S. Pat. No. 7,789,217 is to block off the main culprit creating dust at transfer points, the induced air from material flowing through the transfer chute. This goal was achieved via a two-step approach. In step 1, the material flow is consolidated and guided through the transfer system. In step 2, the cross-section of the chute was filled with material to block off any potential pathway for the induced airflow. However, it was not an adequate solution to the problem of dust control due to automatic control challenges and economics.
There are many controlled flow chutes in today's market, but the chute design of the instant invention can automatically self-adjust to material capacity variations. The adjustable air restrictors cut off the pathway of the induced airflow by closing the gap between the material stream and the chute wall. This design minimizes the induced airflow by directly stopping the air from rushing through the transfer chute and does so without hindering the flow of material. A side effect of keeping the material stream condensed and guided through the transfer chute is a reduction of noise as observed by operation personnel.